1. Field of Invention
The present invention relates to a process and an apparatus for the removal of carbonaceous materials from particles containing such materials.
2. Description of the Prior Art
Fluid catalytic cracking (FCC) processes are used to convert relatively heavy hydrocarbon products obtained from crude oil processing into lighter hydrocarbon products. The catalyst particles used in these processes very quickly become contaminated with carbonaceous materials. These hydrocarbonaceous materials have to be removed in a regeneration process in order to permit reuse of the catalyst particles. In order to regenerate the catalyst, it is contacted with an oxygen-containing gas in a fluidized bed at a temperature suitable to burn off the carbonaceous materials, thus restoring the activity of the catalyst.
Suitable FCC feeds are gas-oils boiling in the range from about 250.degree. C. to about 590.degree. C., especially from about 370.degree. C. to about 540.degree. C. However, at the present moment, there is a tendency to use heavier feeds, such as atmospheric and vacuum residual oils, often mixed with gas-oils. Since these residual feeds usually contain considerable amounts of asphaltenes, which have a high tendency to form coke during the cracking operation, a larger amount of coke will be deposited on the catalyst. This is especially true when only residual feeds are used without mixing them with lighter gas-oil fractions. In regenerating these heavily coked, spent catalyst particles, it may be difficult to burn off enough coke to provide a suitable low concentration of carbon on the regenerated catalyst. To overcome these difficulties, regenerators are suggested in which the coke is burned to carbon monoxide, thus limiting the formation of heat in the regenerator. The carbon monoxide may be used as fuel gas for the production of electricity or steam. Alternatively, a carbon monoxide boiler may be included after the regenerator to complete the combustion of carbon monoxide to carbon dioxide.
A process and an apparatus for the regeneration of spent FCC catalyst particles by burning off the carbon under the formation of carbon monoxide are described in U.S. Pat. No. 4,260,475, issued Apr. 7, 1981. The first stage of this regeneration process is performed in a riser-type regenerator employing an entrained bed of upwardly moving catalyst particles in concurrent flow with regeneration gas. The second stage of the regeneration is performed in a staged fluidized bed of catalyst particles in a net downward movement countercurrent to the regeneration gas. The process of U.S. Pat. No. 4,260,475 has a number of clear advantages over other known regeneration processes, as has been described in its specification.
One disadvantage of the process described in U.S. Pat. No. 4,260,475 is that the riser regenerator has to be of an extremely large height in order to obtain a reasonable removal of carbonaceous materials in the first stage. This is due to the fact that the temperature of the spent catalyst particles is initially too low for a rapid combustion.
An optimum performance of the catalytic cracking process is obtained when the starting reaction mixture of hydrocarbons and catalyst particles in the lower end of the riser reactor has a temperature of 520.degree. C. to 560.degree. C. During the cracking process in the riser reactor, the temperature decreases 20.degree. C. to 30.degree. C. due to the endothermic character of the reaction, thus resulting in a temperature of the spent catalyst particles of 490.degree. C. to 540.degree. C. During the usual stripping of the catalyst, the temperature will fall another 5.degree. C. Thus, the spent catalyst particles introduced into the riser regenerator usually will have a temperature of between 485.degree. C. and 535.degree. C. This temperature is too low for an efficient combustion of the carbonaceous materials deposited on the catalyst particles during the cracking process. Efficient combustion takes place at a temperature of at least 650.degree. C. Therefore, the slowly oxidizing mixture of spent catalyst and oxygen has to be transported over a long distance in the riser regeneration reactor before a sufficiently high temperature is obtained--due to the exothermal oxidation--at which combustion can take place at a reasonable velocity. As a result, riser regeneration reactors of an extremely large height are necessary, leading to an increase in construction costs and a large catalyst inventory, thus resulting in high material, maintenance and operating costs of the unit.
U.S. Pat. No. 4,444,651, issued Apr. 24, 1984, discloses a process for carbo-metallic oil conversion which includes contacting a feed with cracking catalyst containing greater than 600 ppm of nickel equivalents of heavy metal and regenerating the metal-bearing, coked catalyst by burning the coke in a two-stage regeneration zone with oxygen and producing a product gas containing CO.sub.2.
U.S. Pat. No. 4,336,160, issued Jun. 22, 1982, discloses a side-by-side hydrocarbon conversion, two-stage stacked regeneration system whereby the temperature is restricted within the first stage, thereby giving incomplete removal of carbon, and increased in the second stage to allow for the further removal of carbon and the production of a CO.sub.2 rich gas.
U.S. Pat. No. 4,332,674, issued Jun. 1, 1982, discloses a two-stage hydrocarbon conversion-catalyst regeneration process and apparatus which operates with two separate stacked high and low temperature stages and contains a special arrangement to allow for intimate vaporization contact between the residual oil feed and the high temperature catalyst charged to the riser.
U.S. Pat. No. 3,902,990, issued Sept. 2, 1975, discloses a process for regeneration of spent catalyst by introducing the spent catalyst above a dense fluidized bed of catalyst first and then contacting the catalyst with a hot flue gas from the dense fluidized bed followed by an oxygen-containing gas and further contacting it with an additional oxygen-containing gas in the second stage of the process.
U.S. Pat. No. 3,208,831, issued Sept. 28, 1965, discloses an apparatus for the contacting of fluids with fluidized subdivided solid catalyst particles in a reaction chamber followed by a regeneration chamber and then storing the regenerated catalyst.